Hello, guys. I'm trying to deserialize such polymo...

# serialization

[ilyagulya]

Hello, guys.
I'm trying to deserialize such polymorphic json structure using custom KSerializer:

{
  "data": {
    "type1": [
      {
        "id": 1,
        "payload": {
          "fieldA": "123",
          "fieldB": "321"
        }
      }
    ],
    "type2": [
      {
        "id": 1,
        "payload": {
          "fieldC": "789",
          "fieldD": "101112"
        }
      }
    ]
  }
}

And I want to get this as result:

Map<String, List<Item>>

where key is type1, type2 and value list item is:

data class Item(
    val id: Int,
    val payload: ItemPayload
)
sealed class ItemPayload {
    data class PayloadOne(
        val fieldA: String,
        val fieldB: String
    )

    data class PayloadTwo(
        val fieldC: String
        val fieldD: String
    )
}

Right now I'm thinking about to deserialize the JSON to intermediate data class (which has JsonObject instead of ItemPayload), then determine which serializer do I need to use on each particular item and then deserialize the JsonObject to exact ItemPayload based on map key (type1, type2 )
But I don't understand how can I deserialize JsonObject inside the KSerializer .

[janvladimirmostert]

JsonObject has a keys attribute, then i'm assuming you'd need to have a when (key) { "blah" ->
type of setup and then do a get("blah") on that JsonObject to get the sub-JSON which you can then deserialize

Sounds messy, maybe there's a better way

[janvladimirmostert]

pseudo code:

obj.keys.forEach { key ->
when (key) {
"type1" -> result.type1 = JSON.decodeFromJsonElement(Type1.serializer(), obj["type1"])
}

}

[Nikky]

i am not sure if this is stillin the current version of kotlinx.serialization but you can use this Serializer

object MessageSerializer: JsonParametricSerializer<YourType>(YourType::class) {
    override fun selectSerializer(element: JsonElement): KSerializer<out YourType> {
        val obj = element.jsonObject
        if(obj.contains("someKey")) {
            return SomeEvent.seriualizer()
        }
        TODO("add more serializers")
    }
}

[Nikky]

iirc there was some even better solutions in dev at the time which should be available in 1.0

[ilyagulya]

@Nikky thanks. Seems like your way to do that does not fit my requirements, since the type of payload can not be determined in payload deserializer, but in parent deserializers (one for Map<String, List<Item> )

[ilyagulya]

@janvladimirmostert thank you! But it seems that I don't have an access to Json instance inside KSerializer. I can't use JsonConfiguration.Default because I have own configuration.

[Nikky]

iirc you can cast the decoder to JsonDecoder or so, not sure if the name is exactly like so

[ilyagulya]

@Nikky Nice! Thank you! It's JsonInput and it have Json instance, awesome! 🙂

[Nikky]

deserialize the JsonObject to exact ItemPayload based on map key (type1, type2 )

this would also work if you used

@Serializable
data class Data(
  val type1: Item<PayloadOne>
  val type2: Item<PayloadTwo>
)

since it seems like all keys and assosciated types are known, if not you can still make it ignore unknown keys

[ilyagulya]

Thank you 🙂